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EJ501 PLC AND AUTOMATION SYSTEM
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small computers, dedicated to automation
tasks in an industrial environment"
Specialized computer used to control
machines and process.
PLC
Inputs Outputs
Program
A digitally operating electronic apparatus
which uses a programming memory for the
internal storage of instructions for
implementing specific functions such as logic,
sequencing, timing, counting and arithmetic tocontrol through digital or analog modules,
various types of machines or process.
* Nat ional Elect ri cal Manuf actu rers Associat ion of Ameri ca
Basic Characteristic of PLC
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The Hydramatic Division of the General Motors
(GM) Corporation specified the design criteria
for the first programmable controller in 1968.
Their primary goal:-
To eliminate the high costs associated with
inflexible, relay-controlled systems.
Programmable, reusable and reliable.
Basic Characteristic of PLC
The controller had to be designed in modular
form, so that sub-assemblies could be removed
easily for replacement or repair.
The control system needed the capability to passdata collection to a central system.
The system had to be reusable.
The method used to program the controller had
to be simple, so that it could be easily
understood by plant personnel.
Basic Characteristic of PLC
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1968 Programmable concept developed
1969 Hardware CPU controller, with logic instructions, 1 K ofmemory and 128 I/O points
1974 Use of several (multi) processors within a PLC - timers andcounters; arithmetic operations; 12 K of memory and 1024I/O points
1976 Remote input/output systems introduced Microprocessors -based PLC introduced
1980 Intelligent I/O modules developed
Enhanced communications facilities
Enhanced software features (e.g. documentation)Use of personal microcomputers as programming aids
1983 Low - cost small PLCs introduced
1985 on Networking of all levels of PLC, computerand machine using SCADA software.
Basic Characteristic of PLC
Basic Characteristic of PLC
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Less wiring.
Less expensive.
Wiring between devices and relay contacts
are done in the PLC program.
Easier and faster to make changes.
Trouble shooting aids make programming
easier and reduce downtime.
Reliable components make these likely tooperate for years before failure.
Handle much more complicated systems.
Basic Characteristic of PLC
Basic Characteristic of PLC
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Basic Characteristic of PLC
Basic Characteristic of PLC
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Basic Characteristic of PLC
Basic Characteristic of PLC
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Basic Characteristic of PLC
Basic Characteristic of PLC
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Basic Characteristic of PLC
(1)
(2)
Compact PLC
Modular PLC
Smallest and inexpensive Monoprocessor and fieldbus connection Fixed casing Fixed number of I/O (most of them binary) not expandable Typical product: Mitsubishi MELSEC F, ABB AC31, SIMATIC S7 Application Overhead Door control, etc.
Modular construction (backplane) One or multiprocessor system Fieldbus and LAN connection Connection to serial bus I/O Typical products: SIMATIC S5-115, Hitachi H-Serie, ABB AC110 Application Thermocouple input, etc.
3. Rack-PLC
Rack styles PLCs are usually more expensive, expandable, andpowerful than compact or modular PLCs.
The rack provides a power and communication backplane thatgreatly increases the communication rate between theprocessor and the modules as well as allowing some specialtymodules to communicate with each other without the
processor.
The number of available I/O points is also much higher in therack systems.
Racks also allow for redundant
processors for critical systems
such as waste water pumps
or fire control systems.
Basic Characteristic of PLC
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Building structureof PLC
Typically a PLC system has
five basic components.Display/Indicator
Memory
Processor
Power
Supply
Input
Interface
Output
Interface
These are :-
Processor unit
Memory unit
Power supply
unit
Input interface
Output interface Display /indicator
CPUThe processor unit or central processing unit (CPU) is the unit
containing the microprocessor and this interprets the input
signals and carries out the control actions, according to the
program stored in its memory, communicating decisions as a
action signals to the output.
Memory UnitProgram is stored in memory unit, and is used for the
control actions to be exercised by the microprocessor.
1. Random Access Memory (RAM)
read / write
memory will be lost when the power supply disconnected.
2. Read Only Memory (ROM)
read-only memory type.
special equipment is used to delete a memory.
Building structureof PLC
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Display/indicator unitDisplay and indicator unit refers to the internal relay
status display PLC. This can be seen in the Console
programming if the user using the mnemonic code and
programming computer screen if the user using software
programming.
Input InterfaceInputs carry signals from the process into the controller;
they can be input switches, pressure sensors, etc.
These are like the senses and sensors of the PLC.
Output InterfaceOutputs are the devices that the PLC uses to send
changes out to the world. These are the actuator the
PLC can change to adjust or control the process -
motors, lights, relays, pumps, etcBuilding structureof PLC
Building structureof PLC
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Building structureof PLC
PROCESSOR
POWER
SUPPLY
I M
N O
P D
U U
T L
E
O M
U O
T D
P U
U L
T E
PROGRAMMING
DEVICE
From
SENSORS
Pushbuttons,
contacts,
limit switches,etc.
To
OUTPUT
Solenoids,
contactors,
alarmsetc.
Four Steps in the PLC Operations
Input Scan
Scan the state of the Inputs
Program Scan
Processes the program logic Output Scan
Energize/de-energize the outputs
Housekeeping
This step includes communications,
Internal Diagnostics, etc.
The steps are continually repeated - processed in a loop
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Building structureof PLC
Building structureof PLC
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Building structureof PLC
Building structureof PLC
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Building structureof PLC
Programming Unit (PC, Programming Console, Program LoaderUnit, VDU)
i) PC is the commonly used programming device. The softwareallows user to create, edit, document, store and troubleshootprograms. PC communicates with PLC via serial or paralleldata communications link.
ii)Programming Console
Programming Console usually used to program small PLC. Theyare compact, inexpensive and easy to use but are not able todisplay much logic on screen as computer monitor. They oftenused on the factory floor for troubleshooting, modify programand transferring program to multiple machines.
iii)Program Loader Unit
To load the program into the PLC system.
iv)VDU
To processing operations and system status information to bedisplayed by the PLC Building structureof PLC
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Housing Unit
To provide protection to the circuits and components
- the internal components of a PLC.
Secondary Storage Unit
This unit is related to the CPU where all the
programs and information stored.
Power Supply Unit
This unit provides power to the CPU, Input Unit and
Output Unit.Printer Unit
This unit is used to print the program a control
system that is controlled by the PLC, either graphics
or text.Building structureof PLC
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The input devices used with a PLC include
pushbuttons, limit switches, relay contacts,
photo sensors, proximity switches, temperature
sensors, etc. these input devices can be AC or
DC.
The input voltages can be high or low.
The input signals can be analog or digital.
Differing inputs require different input modules.
An input module provides an interface betweeninput devices and a PLCs CPU, which is only use
low DC voltage
The input modules function is to convert
the input signals to DC voltages that are
acceptable to the CPU.
Standard discrete input modules include 24V
AC, 48V AC, 120V AC, 220V AC, 24V DC, 48V
DC, 120V DC, 220V DC, and Transistor-
transistor Logic (TTL) level.
The devices controlled by a PLC include
relays, alarms, solenoids, fans, lights, and
motor starters. These devices may require
different levels of AC or DC voltages
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PLC operated by sending output signals to
external devices called output devices.
Example to output devices are alarm, light,
valve, motor starter etc.
Output modules are used to provide the
electrical connection between the internal
processing unit of PLC and the output devices to
which it provides a signal.
PLC manufactures produce various types ofoutput modules. The modules differ by the types
of electrical signal they produce such as variable
DC voltage, AC voltage and ON/OFF (discrete)
voltage.
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